Polyester films

ABSTRACT

The present invention is a polyester film obtained by blending 10-90 wt % of polyester (A) containing ethylene terephthalate as a main constituent component and 90-10 wt % of crystalline polyester (B) different from this polyester (A), which has a half value width of a recrystallization peak obtained by a differential scanning calorimeter (DSC) by lowering temperature of not more than 0.25. The crystalline polyester (B) is preferably a polyester selected from polybutylene terephthalate (PBT), polytrimethylene terephthalate (PTT) and polyethylene 2,6 naphthalate (PEN). According to the polyester of the present invention, a polyester film superior in mechanical characteristic and design property and having high crystallinity, which can be used alone or adhered to a metal sheet and which is free of whitening even when the film is heat treated at near or not lower than the melting point can be obtained, and further, a polyester film having such superior resistance to whitening (design property), which is not easily damaged, can be obtained.

TECHNICAL FIELD

The present invention relates to a polyester film beneficial as a filmfor packaging of various foods, general industrial use, optical use,electric materials and form processing, as well as a constituentmaterial of a film laminated metal sheet, and the like.

BACKGROUND ART

For example, since a metal can, which is one form of packaging containerfor food and beverage, has many advantages in that long-termpreservation of the contents is possible because it is superior inmechanical strength, packing the contents at a high temperature andsealing as is, as well as easy sterilization treatment such as retorttreatment etc. are available, which leads to high reliability in safetyand hygiene as a packaging container, and further, preservation of thecontents in a heated state is possible and separate collection of canafter use is relatively easy. Thus, various contents have been packedtherein and metal cans have been used in a large quantity in recentyears.

Conventionally, paints containing a thermosetting resin as a maincomponent have been applied to metal cans, because the inner and outersurfaces of a metal can for food and beverage should maintain flavor ofthe contents and prevent corrosion of the metal can, or for the purposeof improving good appearance of the outer surface of the can andprotecting printed surfaces. However, since such metal cans require useof a large amount of solvents during production, they pose problems ofan influence on the environment due to de-solventing during production,problems in hygienic aspects due to residual solvent in the coating,degraded flavor property due to residual oligomer produced by defectivereaction during thermosetting and the like.

To overcome these problems, it has been proposed to laminate a plasticfilm on a metal, and, since more superior aspects have been found interms of adhesive power, heat resistance, mechanical strength, flavorproperty, processing suitability and the like than other resins, of thethermoplastic resins, a polyester film is more often laminated on ametal sheet. As a metal can made by processing such a film-laminatedmetal sheet, what is called a 3-piece can (hereinafter to be abbreviatedas a 3P can) and a 2-piece can (hereinafter to be abbreviated as a 2Pcan) have been proposed. Of these, prevailing of 2P cans has beendesired from the aspect of seamless processing.

As a general production method of 2P can, a typical method comprisesproducing a seamless can by laminating a plastic film, punching alaminated metal sheet with a can making machine and subjecting the sheetto a drawing and ironing step. In such a can making step, a film isrequired to have formability to follow spreading of a metal sheet whilereceiving a shear of drawing and ironing, without being subject to crackand detachment from a metal sheet, and is also required to be free ofwhitening of film due to heating in a can making step, and the like.

In response to such required characteristics, a polyester filmcontaining a polyethylene terephthalate polyester resin having aparticular intrinsic viscosity and a polybutylene terephthalatepolyester resin having a particular intrinsic viscosity has beenproposed.

For example, in U.S. Pat. Nos. 2,882,985, 3,020,731 and the like, byoptimizing semi crystallization temperature and semi crystallizationtime of the polyethylene terephthalate polyester resin and thepolybutylene terephthalate polyester resin to be added, thefollowability a film to deformation of metal and retort whitening aresuppressed.

In JP-A-10-195210, JP-A-10-110046 and the like, moreover, followabilityof a film to deformation of metal and flavor property by heat treatment(crystallization treatment) after lamination are improved by optimizingthermal property and planar orientation of a polyethylene terephthalatepolyester resin and a polybutylene terephthalate polyester resin to beadded. In so doing, when transesterification has proceeded too muchbetween two kinds of polyesters, the crystallinity of the film cannot beincreased. Therefore, for example, melting time of resin during filmforming and the amount of heat applied to a film during subsequentstretching and heat treatment step of the film are designed to bedecreased.

Moreover, JP-A-2002-179892, JP-A-2002-321277 and the like propose apolyester film which, though it is a blend film of a polyethyleneterephthalate (PET) polyester resin and a polybutylene terephthalate(PBT) polyester resin, is alleged to be able to maintain heat sealproperty and formability because PET phase and PBT phase compriseindependent crystals. This film is considered to be usable for formingtransfer, forming container, metal adhering and the like, and forming afilm by extruding a blended resin by a vent-type extruder to give a filmhas been proposed to make PET phase and PBT phase independent crystals.

However, according to the consideration of the present inventors, it hasbeen found that, in conventional blend type polyester films proposedabove, when a film is melted at a near melting point or not lower thanthe melting point, adhered to a metal and the like and then cooled,problems occur in that the film is whitened and the design of the filmis degraded. In addition, a problem has been found that the film on ametal sheet is damaged easily.

DISCLOSURE OF THE INVENTION

In view of the above-mentioned situation, the present invention aims atproviding a polyester film superior in mechanical characteristic anddesign property, which can be adhered to a metal sheet and which is freeof whitening even when the film is heat treated at near or not lowerthan the melting point. Moreover, the present invention aims atproviding a polyester film having the above-mentioned superiorcharacteristics, which is not easily damaged after laminating on a metalsheet.

The present inventors have studied the cause of the whitening of aconventional polyester film made from a blend of a polyethyleneterephthalate polyester resin and a polybutylene terephthalate polyesterresin, which occurs along with lowering of the temperature after meltingat near or not lower than the melting point, and considered that suchwhitening occurs because the size of a crystal produced in temperaturelowering process after melting of the film is too big, and the increasedcrystal size of the film is affected by the dispersion state of thefilm-constituting polyester (i.e., PET polyester resin and PBT polyesterresin), and further proceeded with the investigation from such aspect,which resulted in the completion of the present invention.

Accordingly, the present invention relates to

(1) a film made from a polyester resin composition comprising 10-90 wt %of polyester (A) comprising ethylene terephthalate as a main constituentcomponent and 90-10 wt % of crystalline polyester (B) different fromsaid polyester (A), which film shows a half value width ofrecrystallization peak obtained by a differential scanning calorimeter(DSC) by lowering temperature of not more than 0.25,(2) the polyester film of the above-mentioned (1), wherein the polyesterresin composition comprises 10-70 wt % of polyester (A) comprisingethylene terephthalate as a main constituent component and 90-30 wt % ofcrystalline polyester (B), and the crystalline polyester (B) is apolyester selected from polybutylene terephthalate (PBT),polytrimethylene terephthalate (PTT) and polyethylene-2,6-naphthalate(PEN),(3) the polyester film of the above-mentioned (1) or (2), wherein a peaktemperature (Tc2) of a recrystallization peak is not less than 180° C.,(4) the polyester film of any of the above-mentioned (1)-(3) wherein thepolyester film has a reduced viscosity of not less than 0.80,(5) the polyester film of any of the above-mentioned (1)-(4), which isused for laminating on a metal sheet, and(6) the polyester film of any of the above-mentioned (1)-(4), which isused for form processing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a chart showing a recrystallization peakof a polyester film during a temperature decrease by a differentialscanning calorimeter (DSC).

For explanation of the symbols in the FIGURE, 10 is a recrystallizationpeak, 11 is a peak top (vertex), L1 is a base line, h is a height fromthe base line to the peak top, and l is a temperature width at ½ h.

DETAILED DESCRIPTION OF THE INVENTION

The polyester (A) comprising ethylene terephthalate as a mainconstituent component (hereinafter to be also referred to as polyesterA) in the present invention refers to a polyester comprising ethyleneterephthalate in not less than 80 mol %, preferably not less than 90 mol%, of the polyester component.

As used herein, the polyester is a polymer comprising a dicarboxylicacid component and a glycol component, and the dicarboxylic acidcomponent optionally selected from dicarboxylic acid components otherthan terephthalic acid, such as aromatic dicarboxylic acids (e.g.,naphthalene dicarboxylic acid, diphenyl dicarboxylic acid,diphenylsulfon dicarboxylic acid, diphenoxyethane dicarboxylic acid,5-sodiumsulfoisophthalic acid, phthalic acid and the like), aliphaticdicarboxylic acids (e.g., oxalic acid, succinic acid, adipic acid,sebacic acid, dimer acid, maleic acid, fumaric acid and the like),alicyclic dicarboxylic acids (e.g., cyclohexane dicarboxylic acid andthe like), oxycarboxylic acids (e.g., p-oxybenzoic acid and the like)and the like may be copolymerized. In addition, a glycol componentoptionally selected from glycol components other than ethylene glycol,such as aliphatic glycols (e.g., trimethylene glycol, butanediol,pentanediol, hexanediol, neopentyl glycol and the like), alicyclicglycols (e.g., cyclohexanedimethanol and the like), aromatic glycols(e.g., bisphenol A, bisphenol S and the like) and the like may becopolymerized. Two or more kinds of these dicarboxylic acid componentsand glycol components may be used in combination.

The melting point of polyester (A) is preferably 240° C. -265° C.

The reduced viscosity of polyester (A) is preferably 0.55-0.90, morepreferably 0.58-0.80. When the reduced viscosity becomes smaller thanthis range, a film having mechanical strength standing practical use isdifficult to obtain, whereas when it exceeds this range, the filmunpreferably loses heat press sealability to a metal sheet.

The crystalline polyester (B) (hereinafter to be also referred to aspolyester B) in the present invention refers to one or more kinds ofpolyesters selected from polybutylene terephthalate (PBT) polyester,polyethylene-2,6-naphthalate (PEN) polyester, polytrimethyleneterephthalate (PTT) polyester, polyhexamethylene terephthalate (PHT)polyester and polypentamethylene terephthalate (PPT) polyester, which ispreferably polybutylene terephthalate (PBT) polyester,polyethylene-2,6-naphthalate (PEN) polyester, polytrimethyleneterephthalate (PTT) polyester, polyhexamethylene terephthalate (PHT)polyester or polypentamethylene terephthalate (PPT) polyester. As usedherein, polyester is a polymer comprising a dicarboxylic acid componentand a glycol component. These crystalline polyesters are all preferablyhomopolyesters (i.e., polybutylene terephthalate (PBT),polyethylene-2,6-naphthalate (PEN), polytrimethylene terephthalate(PTT), polyhexamethylene terephthalate (PHT), polypentamethyleneterephthalate (PPT)), and may be copolymerized polyesters wherein thehomopolyester is further copolymerized with other dicarboxylic acidcomponent and/or glycol component. The dicarboxylic acid component assuch a copolymerizable component includes, for example, any dicarboxyliccomponents selected from aromatic dicarboxylic acids such as diphenyldicarboxylic acid, diphenylsulfon dicarboxylic acid, diphenoxyethanedicarboxylic acid, 5-sodiumsulfoisophthalic acid, phthalic acid and thelike, aliphatic dicarboxylic acids such as oxalic acid, succinic acid,adipic acid, sebacic acid, dimer acid, maleic acid, fumaric acid and thelike, alicyclic dicarboxylic acids such as cyclohexane dicarboxylic acidand the like, oxycarboxylic acids such as p-oxybenzoic acid and thelike, and the like, and as the glycol component, any glycol componentselected from aliphatic glycols such as ethylene glycol, trimethyleneglycol, butanediol, pentanediol, hexanediol, neopentyl glycol and thelike, alicyclic glycols such as cyclohexanedimethanol and the like,aromatic glycols such as bisphenol A, bisphenol S and the like, and thelike, where two or more kinds of these dicarboxylic acid components andglycol components may be each used in combination. The amount ofcopolymerization of these dicarboxylic components and/or glycolcomponents is in such a range that does not impair high crystallinity ofthe polyester, which is generally not more than 20 mol % of the entirepolyester.

The melting point of polybutylene terephthalate (PBT) polyester ispreferably 215-235° C., the melting point ofpolyethylene-2,6-naphthalate (PEN) polyester is preferably 260-275° C.,the melting point of polytrimethylene terephthalate (PTT) polyester ispreferably 230-245° C., the melting point of polyhexamethyleneterephthalate (PHT) polyester is preferably 150-170° C., and the meltingpoint of polypentamethylene terephthalate (PPT) polyester is preferably130-150° C.

In the present invention, the crystalline polyester (B) is particularlypreferably selected from polybutylene terephthalate (PBT) polyester,polytrimethylene terephthalate (PTT) polyester andpolyethylene-2,6-naphthalate (PEN) polyester, and specificallypreferably polybutylene terephthalate (PBT) polyester orpolytrimethylene terephthalate (PTT).

In the present invention, the reduced viscosity of crystalline polyester(B) is preferably 0.80-2.20, more preferably 0.85-1.50. When the reducedviscosity is lower than this range, a film having mechanical strengthcapable of standing practical use is difficult to obtain, whereas whenit exceeds this range, the film unpreferably loses heat presssealability to a metal sheet.

In the polyester film of the present invention, the mixing ratio ofpolyester A and polyester B (A/B) is generally 10-90 wt %/90-10 wt %,preferably 10-70 wt %/90-30 wt %, more preferably 35-65 wt %/65-35 wt %.When the amount of polyester A exceeds such range (when the amount ofpolyester B is small), the form processability of the film is degraded,and particularly, when a laminated metal sheet, wherein this film islaminated on a metal sheet, is used for can making, a can making failureoccurs and the film easily breaks. When the amount of polyester B ishigh (when the amount of polyester A is small), whitening of the film,which occurs when the film is melted to a temperature near or not lowerthan the melting point thereof and then cooled, cannot be suppressedsufficiently.

FIG. 1 is a simplified view of a chart showing recrystallization peak ofa crystalline polyester film during a temperature decrease by adifferential scanning calorimeter (DSC). By the “half value width of arecrystallization peak obtained by a differential scanning calorimeter(DSC) by lowering temperature” in the present invention is meanttemperature width l divided by height h (l/h), wherein h is a heightfrom a base line L1 to a peak top 11 in a recrystallization peak (chart)10 of a polyester film as measured by a differential scanningcalorimeter (DSC), and l is a temperature width at a height of 0.5 h.The rate of crystallization during temperature decrease in arecrystallization process of a polyester film can be known from the halfvalue width (l/h) of such recrystallization peak, wherein the smallerthe value is, the quicker the (heat generation) thermolysis occurs,indicating a rapid crystallization rate.

In other words, the present invention is based on the finding that, whenpolyester A and polyester B are blended at the above-defined mixingratio and the obtained film forms a dispersion state of polyester suchthat the half value width (l/h) of a recrystallization peak obtained bya differential scanning calorimeter (DSC) by lowering temperaturebecomes not ore than 0.25, the film is free of whitening even when it ismelted to a near or not lower than the melting point and then cooled. Afilm having a half value width of crystallization peak of not more than0.25 shows high crystallization rate during cooling process from themelting point in temperature decrease, and since microcrystal that doesnot contribute to the diffusion of visible light is formed in a largeamount inside the film, the film is not whitened and maintainsprocessing characteristic. When the half value width (l/h) of therecrystallization peak is greater than 0.25, crystallization rate isslow in the cooling process of decreasing temperature from the meltingpoint, and microcrystals grow to the level that diffuses the visiblelight, which in turn causes whitening of the film.

In the present invention, the half value width (l/h) of such arecrystallization peak is preferably not more than 0.22, more preferablynot more than 0.20. When a polyester film shows such preferable halfvalue width (l/h) of recrystallization peak, more superior resistance towhitening is shown.

In the present invention, a polyester film showing a half value width ofa recrystallization peak obtained by a differential scanning calorimeter(DSC) by lowering temperature of not more than 0.25 can be achieved bycontrolling the dispersion state of polyester A and polyester B in apolyester mixture during a film production process.

Conventionally, when a blend type polyester film is produced, polyesterchips to be blended are generally mixed (dry blend) before feeding intoan extruder, and such chip mixture is fed into an extruder andmelt-mixed. However, according to this method, plural polyester chipssubstantially almost simultaneously start melting. Therefore, pluralpolyesters to be blended are melt-mixed for a long time, and compatiblepolyesters of PET and PBT become a finely dispersed state. Theconventional polyester films comprising a blend of a PET polyester resinand a PBT polyester resin explained under the section of Prior Art areall produced by this method, and the problem of whitening, which occursin conventional polyester films comprising a blend of a PET polyesterresin and a PBT polyester resin when laminating the film on a metal andthe like upon melting to a near or not lower than the melting point, isconsidered to occur because both polyesters (PET polyester resin, PBTpolyester resin) constituting the film are not copolymerized, and evenif they are dispersed in substantially mutually independent phases,since they are in a finely dispersed state (namely, subject to aninfluence of each other due to the state of fine dispersion),independent crystallinity of the both cannot be maintained. As a result,crystallization of the film does not proceed easily, thus leading to thewhitening of the film. For example, therefore, when a blend film of PET(melting point 255° C.) and PBT (melting point 220° C.) is to beproduced by a single extruder having a uniaxial or twin screw, thetemperature of the extruder needs to be set to a temperature not lowerthan the melting point of PET, because PET has a high melting point, andin view of the stability and the like of the production, it is generallyset to not lower than 280° C. However, when PBT is heated to atemperature of not less than 280° C., decomposition that starts fromaround 260° C. proceeds more quickly, and the molecular weight decreasesdue to the heat of the extruder. As a result, compatibility with PETbecomes still higher, resulting in a tendency toward uniformization tobecome finely dispersed. Thus, the two tend to copolymerize(transesterification) in the melt-mixing process, and even if they arenot copolymerized, the individual crystallinity is impaired and theybecome highly susceptible to an influence of each other, which in turnmakes it difficult to maintain independence of each crystallinity, thusdecreasing crystallization rate of the film.

The polyester film of the present invention is applied to a variety ofuses such as for packaging of various foods, general industrialpurposes, optical use, electric materials, metal lamination, formprocessing and the like. Therefore, not only the occurrence of whiteningshould be avoided even if the film is melted at near or not lower thanthe melting point, but also superior formability (formability of filmalone and formability in laminating processing on metal sheet) andsuperior flavor property should be achieved, and further, whitening offilm after forming should be avoided. To meet these goals, thecrystallization rate and crystallinity of the film should be controlledby sufficiently suppressing copolymerization ratio of polyester A andpolyester B (transesterification of the two). In the present invention,for example, polyester A chip and polyester B chip are not mixed (dryblended) but fed into independent extruders and melted therein,polyester A and polyester B melted in these independent extruders andprior to extrusion thereof from a die are mixed and led to a die, whichis followed by steps of melting, extrusion and film forming, therebyshortening the contact time of polyester A and polyester B in a moltenstate, and forming a film while maintaining a “crudely mixed” dispersionstate wherein polyester A and polyester B are each dispersed in thestate of a relatively large crystal phase. A film, wherein polyester Aand polyester B are each dispersed in the state of a relatively largecrystal phase, maintains independence of crystallinity of each polyester(polyester A and polyester B), allowing rapid progress ofcrystallization of the film, whereby the half value width (l/h) ofrecrystallization peak of the film is considered to become not more than0.25.

In this way, the polyester film of the present invention can be producedby independently melting polyester (A) comprising ethylene terephthalateas a main constituent component and crystalline polyester (B) differentfrom the polyester (A), mixing them in a molten state at a predeterminedmixing ratio, and forming the mixture. In conventional generalproduction of a polyester film, a material (polyester) constituting afilm having a single layer is fed into a single extruder, melted,extruded and formed into a film. The polyester films described in patentreferences exemplarily shown in the section of Prior Art comprise two ormore kinds of different starting materials (polyesters), but a singleextruder is used to melt and mix the two or more kinds of differentstarting materials (polyesters) all at once. This is assumed to resultfrom the consideration of stability and economical aspect offilm-forming operation, due to which the production of a higher qualityfilm seems to have been difficult. In contrast, according to the presentinvention, it has been found that a film having improved quality can beobtained while maintaining the stability of film-forming by separatelymelting different starting materials (two or more kinds of polyesters),mixing them in a molten state and extruding the mixture. In the presentinvention, while a method comprising melting individual startingmaterials (two or more kinds of polyesters) in separate extruders andmixing them in a molten state (a method using two or more extruders inparallel) is preferable, production by a single extruder is alsopossible. In the case of production by a single extruder, however, anextruder having a double flight type compression part (compression zone)of a screw needs to be used. As used herein, by the “double flight type”is meant a constitution wherein a compression part (compression zone) ofa screw is a double helical structure with a subflight having an outerdiameter a little smaller than the outer diameter of the main flightinstalled between the main flights, which is characterized in that thesubflight separates a solid phase and a molten part of the polymer. Asan extruder having a screw with such a double flight type compressionpart (compression zone), for example, UB series (product name) ofMITSUBISHI HEAVY INDUSTRIES, LTD., BARR series (product name) of TheJapan Steel Works, LTD. and the like can be mentioned. As long as anextruder comprises a screw having a double flight type compression part,a single extruder can also produce the polyester film of the presentinvention, assumedly because of the following reasons. That is, forexample, when PET and PBT are blended, PBT that starts melting first andPET that remains solid at that time point are separated in the formerflight of the compression part of an extruder, which shortens thecontact time of PET and PBT in a molten state and achieves “crudemixing”. In this case, the single extruder is preferably a rapidcompression type having a smaller compression ratio (not more than 2.0).This is because, when a slow compression type having a greatercompression ratio (more than 2.0) is used, even if the apparenttemperature can be set low, the amount of self heat generation at acompression part (compression zone) of the extruder increases, and thetemperature of the resin becomes the temperature set or higher thanthat, and therefore, when PTT, PBT, PHT and the like having aparticularly low melting point are used from crystalline polyester resin(B), decomposition thereof unpreferably tends to occur easily.

In the present invention, moreover, a rapid compression type extruderrefers to one containing a compression part (compression zone) having anL/D of less than 25, preferably less than 20, more preferably less than15, and a slow compression type extruder refers to one containing acompression part (compression zone) having an L/D of not less than 25.As used herein, L/D is a ratio of an effective length (L) and an outerdiameter (D) of a screw of a compression part (compression zone),wherein the outer diameter (D) is an average outer diameter of theeffective length of a screw.

As mentioned above, while the polyester film of the present inventioncan be produced by a single extruder, since a single extruder permits anarrow range of tolerance of mixing conditions, a method comprisingseparately melting plural polyesters in individual extruders ispreferable. To be specific, as an extruder to be used for separatelymelting polyester A and polyester B, an extruder having a compressionratio of 1.1-3.1 (preferably 1.5-2.8) and an L/D of 20-35 (preferably25-30) is preferable. Such extruder may be a uniaxial extruder or abiaxial extruder. As used herein, the L/D is a ratio of an effectivelength (L) and outer diameter (D) of a screw of an extruder, wherein theouter diameter (D) is an average outer diameter of the effective lengthof a screw. The compression ratio is calculated by (groove depth of feedzone)÷(groove depth of metering zone). The groove depth of the feed zoneand metering zone is calculated from the outer diameter and groovediameter of the screw, respectively.

As a machine to mix the separately melted polyester A and polyester B,for example, uniaxial extruder, biaxial extruder, dynamic mixer, staticmixer (Noritake Co., Limited) and the like can be mentioned, withpreference given to uniaxial extruder and static mixer.

As an extruder to mix the melted polyester A and polyester B, sinceuniform mixing of the melted polyester A and melted polyester B to thedegree they become compatible with each other (transesterification) isnot preferable, one having a small compression ratio is preferably used.To be specific, one having a compression ratio of 1.1-3.8 (preferably1.3-3.0) is preferable. It is also preferable to use an extruder havingan L/D of 20-35 (preferably 25-30), from the aspect of crude mixing ofpolyester A and polyester B, and use of an extruder having an L/D of thecompression part of 5-25, preferably 10-20, is preferable. The L/Dcompression ratio here means the same as the aforementioned, and L/D ofthe compression part (compression zone) is the ratio of length (L) andouter diameter (D) of the compression part (compression zone) of ascrew.

The extruding conditions (forming conditions) of the molten resinmixture are preferably not more than 265° C. of resin temperature and donot create an area of not less than 275° C. (preferably not less than270° C.) in the temperature setting from the cylinder part to T-die.This is out of the possibility that, when these conditions are not met,a higher temperature increases compatibility of polyesters A, B, whichin turn results in a failure to suppress whitening of the object film,or causes decrease of the viscosity (molecular weight) of the film,thereby lowering the scratch resistance of the produced film.

When polyester A and polyester B have higher compatibility,transesterification occurs between polyester A and polyester B to form acopolymer of polyester A and polyester B, an ethylene terephthalatestructure, which is the main component structure of polyester A, becomesat random, thus impairing the characteristic rigidity thereof, as wellas butylene terephthalate structure, trimethylene terephthalatestructure, hexamethylene terephthalate structure, pentamethyleneterephthalate structure or ethylene-2,6-naphthalate structure, which isthe main component structure of polyester B, becomes at random, thusimpairing the characteristic high crystallinity. As a result,crystallization rate becomes slow and coarse spherical crystal thatcould cause whitening of the film easily occurs.

While means are known to suppress side reaction such astransesterification and the like in a film composition containingpolyethylene terephthalate polyester and other crystalline polyesters,for application in the industrial film production, a method comprisingadding an organic phosphorus compound (catalyst) is preferable forsuppression of copolymerization of polyester A and polyester B. In thepresent invention, too, addition of such organic phosphorus compound(catalyst) enhances the transesterification suppressing effect.

In the present invention, such organic phosphorus compound (catalyst)having a melting point of not less than 200° C. is preferable from theaspect of stability in an extruder (mixing machine). For example,dimethyl methylphosphonate, diphenyl methylphosphonate, dimethylphenylphosphonate, diethyl phenylphosphonate, diphenylphenylphosphonate, dimethyl benzylphosphonate, diethylbenzylphosphonate, diphenylphosphinic acid, methyl diphenylphosphinate,phenyl diphenylphosphinate, phenylphosphinic acid, methylphenylphosphinate, phenyl phenylphosphinate, diphenylphosphine oxide,methyldiphenylphosphine oxide, triphenylphosphine oxide and the like canbe mentioned. Of these, those having a molecular weight of not less than200 are particularly preferable. One or more kinds of such organicphosphorus compounds can be used. The amount of addition variesdepending on the kind of the phosphorus compound, but it is generallypreferably about 0.01-0.3 wt % of the total amount of the polyester tobe blended. It is also preferable to have such organic phosphoruscompound prekneaded with a resin, and prekneading with at leastpolyester A chip is more preferable.

When the polyester film is used for food, such as leverage can and thelike, the organic phosphorus compound needs to be used in a compound andan amount meeting the standard of FDA (U.S. Food and DrugAdministration), Japan Hygienic Olefin And Styrene Plastics Associationand the like.

While a production method of the polyester film of the present inventioncomprising separately melting polyester A and polyester B and mixingthem in a molten state has been explained in detail in the above, if afilm comprising polyester A and polyester B dispersed in a state ofrelatively large crystal phase (i.e., “crude mixture”) can be formed byother methods, whereby the half value width of a recrystallization peakduring a temperature decrease by a differential scanning calorimeter(DSC) of the film can be controlled to not more than 0.25, suchpolyester film is also encompassed in the polyester film of the presentinvention.

In the polyester film of the present invention, the temperature (Tc2) ofa recrystallization peak during decreasing temperature by differentialscanning calorimeter (DSC) is preferably not less than 180° C., morepreferably not less than 185° C., further preferably not less than 190°C., most preferably not less than 192° C. When the temperature of therecrystallization peak is such temperature, the crystallization rate ofthe film in the cooling process at decreasing temperature from themelting point becomes faster, and more preferable results can beobtained in the suppression of whitening of a film. When therecrystallization peak temperature is too high, such polyester filmshows degraded form processability. Particularly when laminated on ametal sheet and the laminated metal sheet is subjected to can making, acan making failure occurs and the film is easily broken. Thus, arecrystallization peak temperature of not more than 250° C. ispreferable.

As mentioned above, the suppression of transesterification betweenpolyester resins to be blended acts favorably for suppression ofwhitening, but in the present invention, dispensability and/orcompatibility of polyester A and polyester B are/is particularlydegraded, in other words, each polyester is “crudely mixed” bydispersion in relatively large crystal phase, whereby a polyester filmsuperior in anti-whitening property, which shows the object half valuewidth (l/h) of a recrystallization peak obtained by a differentialscanning calorimeter (DSC) by lowering temperature of not more than0.25, can be obtained. Therefore, addition of a material that becomes anucleating agent for crystallization affords more preferable results. Assuch nucleating agent, any of organic fine particles and inorganic fineparticles can be used, which are exemplified by silica, kaolin, calciumcarbonate, titanium dioxide, polyethylene is glycol and the like, withpreference given to talc. The amount of addition is suitably about0.0001-0.1 wt % of the entire film.

The reduced viscosity (ηsp/c) of the polyester film of the presentinvention is preferably not less than 0.80, more preferably not lessthan 0.85, further preferably not less than 0.90, most preferably notless than 0.95. When it is less than 0.80, a film after lamination on ametal sheet has insufficient hardness, which causes easy damage duringprocessing, deformation of aluminum and steel sheet during can making,and partial destruction. When the reduced viscosity satisfies suchvalues and recrystallization peak temperature (Tc2) satisfies theaforementioned numerical figures, the film comes to have furtherimproved scratch resistance.

As long as the polyester film of the present invention meets theabove-mentioned conditions, it can be produced by general film formingfacility by inflation methods, simultaneous biaxial drawing methods,sequential biaxial drawing methods and the like. In addition, a meltextruded sheet may be used without drawing or subjected to uniaxialdrawing.

In the present invention, moreover, a heat treatment is preferablyapplied at 140-270° C., preferably 140-200° C. after completion ofdrawing. In this case, a heat treatment while relaxing by not less than2% in the longitudinal and/or transverse direction improves adhesivenessof the film to a metal sheet and can making operability.

The polyester film of the present invention preferably has a thicknessof 3-1000 μm, more preferably 5-70 μm. Generally, a lubricant is addedto a polyester and formed to give a film. As such lubricant, inorganiclubricants such as silica, kaolin, clay, calcium carbonate, calciumterephthalate, aluminum oxide, titanium oxide, calcium phosphate and thelike, organic lubricants such as silicone particles and the like can bementioned, with preference given to inorganic lubricants. These areadded in a proportion of 0.01-5 wt %, preferably 0.02-0.2 wt % of thetotal weight of the polyester starting materials to be blended. Thepolyester film of the present invention can contain additives asnecessary, besides the lubricant, such as stabilizer, coloring agent,antioxidant, antifoaming agent, antistatic agent, and the like.

When a film laminated metal sheet is prepared using the polyester filmof the present invention, a roller or a metal sheet is heated to150-270° C., the metal sheet and a polyester film are adhered via theroller, cooled rapidly, and at least a surface layer of the polyesterfilm in contact with the metal sheet is melted and welded. Thelamination rate is generally 1-200 m/min, preferably 2-150 m/min.

The polyester film of the present invention is used as a film forpackaging of various foods, general industrial purposes, optical use,electric materials and form processing. To be specific, it is preferablefor general packaging, antistatic purpose, gas barrier, metallamination, heat sealing, antifog, metal vapor deposition, easy tearing,easy opening, bag making and packaging, retort packaging, boilpackaging, drug packaging, easy adhesion, magnetic recording, capacitor,ink ribbon, transfer, adhesive label, stamping foil, gold and silverthreads, tracing material, release, shrinkable film and the like, andparticularly preferable for form processing (forming material forforming a film alone to give object article) and metal lamination(constituent material for film laminated metal sheet).

In the present specification, the properties and tests were measured andevaluated according to the following methods.

1. Reduced Viscosity

Reduced Viscosity (ηsp/C)

Polymer (0.125 g) was dissolved in 25 ml of phenol/tetrachloroethane=6/4(weight ratio) and measured at 25° C. using a Ubbelohde viscometer,wherein the unit is dl/g.

2. Melting Point of Polyester, Crystallization Temperature of Film andHalf value Width of Crystallization Peak

Using DSC 3100S manufactured by Rigaku Corporation, a polyester film wasplaced in a sample pan, a lid was put on the pan, the temperature wasraised under a nitrogen gas atmosphere from room temperature to 280° C.at a temperature rise rate of 20° C./min, and the temperature of thepeak top of the melting peak that appears by temperature rise(temperature of peak top on low temperature side (TmL), temperature ofpeak top on high temperature side (TmH)) was taken as the melting pointof the polyester blend.

A sample that reached 280° C. was maintained as it was for 1 min,thereafter cooled to room temperature at a rate of 20° C./min and acrystallization peak (chart) was measured wherein peak top temperaturewas Tc2. A half value width (l/h) was calculated by dividing temperaturewidth l at a height of 0.5 h by height h, wherein a height from a baseline to a peak top in this peak (chart) was h.

3. Whitening of Film

A film was laminated on an aluminum sheet under the followingconditions, and left standing in a gear oven for 1 min at 280° C. and290° C. (two levels). Then air at 25° C. was blown against the filmsurface of the laminated sheet at wind speed of 20 m/min for cooling andthe film was visually evaluated.

(Laminate Conditions)

-   -   laminate temperature: 220° C.    -   linear pressure: 10N/cm        (Evaluation)

-   ⊙ . . . No whitening was observed at all before and after heat    treatment, and the (level of) gloss of the film was high.

-   ◯ . . . Whitening was observed somewhat, but the (level of) gloss of    the film surface was high.

-   Δ . . . Whitening was observed and the (level of) gloss of the film    surface was low.

-   X . . . Whitening was remarkable and the (level of) gloss of the    film surface was low.    4. Hardness of Film

A film surface of the laminated sheet heated and cool-treated in 3. wasrubbed hard with a pencil having a sharp point, and evaluated based onthe highest hardness of a pencil that did not give a scratch then.

5. Can Making Property

A film was laminated on an aluminum sheet under the conditions of 3.,treated at 240° C., a can was formed, the presence of breakage of thefilm such as detachment, cut, crack and the like was observed visuallyand with a fluorescence microscope (magnification ×80) and evaluatedbased in the following standard.

-   ⊙ . . . Of 100 cans, no breakage in not less than 95 cans.-   ◯ . . . Of 100 cans, no breakage in 80-94 cans.-   Δ . . . Of 100 cans, no breakage in 70-79 cans.-   X . . . Of 100 cans, some breakage in not less than 31 cans.

The present invention is explained in more detail by referring toExamples.

EXAMPLE 1

As polyester A, polyethylene terephthalate (reduced viscosity 0.75,catalyst was germanium dioxide) containing in advance 2000 ppm of silica(Silysia 310 manufactured by Fuji Silysia Chemical Ltd.) added duringpolymerization was fed into a 60 mmφ extruder I (L/D=29, compressionratio 4.2) and melted at 275° C. As polyester B, 300 ppm of polybutyleneterephthalate (1200S manufactured by Toray Industries, Inc., reducedviscosity 1.30) and an organic phosphorus compound (Adeka Stab PEP-45manufactured by Asahi Denka Co., Ltd.) were fed into a different 60 mmφextruder II (L/D=29, compression ratio 4.2) and melted at 240° C.Thereafter the molten products from extruder I and extruder II were ledin a molten state to a 90 mmφ extruder III (L/D=25, L/D of compressionpart=12, compression ratio 1.5) such that the amount ratio becomes(I/II)=4/6 (weight ratio), fed, mixed, melted and extruded from a T-dieto give a 200 μm thick non-oriented sheet. The temperature then of thecylinder part and the filter part (200 mesh) of extruder III was set to260° C., the temperature from the tip of the screw of the extruder toT-die was set to 255° C. and the temperature of the resin that came outfrom a T-die was set to 257° C. In addition, the pressure of the resinimmediately before entering the T-die was set to 8.8 MPB (90 kgf/cm²).

This non-oriented sheet was led to a roll drawing machine, drawn 3.3times in the longitudinal direction at 70° C., further drawn 3.5 timesin the transverse direction on a tenter at 95° C. and subjected tothermosetting at 150° C. in situ while relaxing by 3% in the transversedirection in the tenter to give a 17 μm thick film.

EXAMPLE 2

In the same manner as in Example 1 except that the starting material ofextruder II was polybutylene terephthalate (1200S manufactured by TorayIndustries, Inc., reduced viscosity 1.30) prekneaded with 300 ppm of anorganic phosphorus compound (Adeka Stab PEP-45 manufactured by AsahiDenka Co., Ltd.) and 500 ppm of talc, as polyester B, a film wasobtained. The temperature of the resin that came out from a T-die was258° C.

COMPARATIVE EXAMPLE 1

In the same manner as in Example 1 except that the temperature of thefilter part of extruder III was set to 285° C., the temperature from thetip of screw of the extruder III to T-die was set to 282° C. and thetemperature of the resin that finally came out from a T-die was set to266° C., a film was obtained.

EXAMPLE 3

In the same manner as in Example 1 except that the compression ratio ofextruder III was set to 4.0, a film was obtained. While the conditionsof temperature setting and the like were completely the same as inExample 1, the temperature of the resin that came out from a T-die was263° C.

COMPARATIVE EXAMPLE 2

In the same manner as in Example 1 except that the polyesters A, B andorganic phosphorus compound used in Example 1 were fed as pellets intoextruder III (L/D=25, compression ratio 4.0) such that the compositionratio became the same as in Example 1, and the temperature conditionswere the same as in Example 1, a film was obtained. While thetemperature conditions were the same as in Example 1, the temperature ofthe resin that came out from a T-die was 265° C.

EXAMPLE 4

In the same manner as in Example 1 except that polytrimethyleneterephthalate (reduced viscosity 0.98) was used instead of polybutyleneterephthalate, a film was obtained. The temperature of the resin thatcame out from a T-die was 260° C.

EXAMPLE 5

In the same manner as in Example 1 except that a static mixer (N20manufactured by Noritake Co., Limited, number of elements 12, cylindertemperature 258° C.) was used instead of extruder III, a film wasobtained. The temperature of the resin that came out from a T-die was258° C.

EXAMPLE 6

After obtaining a 38 μm thick film by a method similar to that inExample 1, the film was formed at 90° C. using a metal mold to give asurface cover (depth 5 mm×width 50 mm×length 50 mm) of liquid crystalfor mobile phone. It was confirmed to be fine, free of whitening andfine for form processing.

COMPARATIVE EXAMPLE 3

In the same manner as in Comparative Example 2 except that polyethyleneterephthalate containing true spherical silica having an averageparticle size of 1.5 μm and polybutylene terephthalate were mixed (dryblend) at 42:58 (weight ratio) and fed into extruder III (L/D=25, L/D ofcompression part=12, compression ratio 4.0) as pellets and thetemperature was exclusively set to 290° C., a 12 μm film was obtained.The temperature of the resin that came out from a T-die was 290° C. Themelting point peak seen was only one by DSC, possibly because the resintemperature was high.

COMPARATIVE EXAMPLE 4

Polyethylene terephthalate (30 wt %, reduced viscosity 0.76 (intrinsicviscosity 0.67)) containing 0.1 wt % of silica having an averageparticle size of 1.0 μm and polybutylene terephthalate (Novadure 5009ASmanufactured by Mitsubishi Engineering-Plastics Corporation, 70 wt %)were fed into an extruder. This extruder had a slow compression screw of75 mmφ, L/D=45, compression ratio 3.5, L/D of compression part=30. Inthe same manner as in Example 1 except that the temperature wasexclusively set to 265° C., a 12 μm film was obtained. The temperatureof the resin that came out from a T-die was 290° C. While two meltingpoint peaks were seen by DSC, the half value lo width of therecrystallization peak at a decreasing temperature was broad andwhitening was seen in many points, possibly due to the high compressionratio and slow compression screw.

COMPARATIVE EXAMPLE 5

Polyethylene terephthalate (50 wt %, reduced viscosity 0.80 (intrinsicviscosity 0.70)) containing 0.1 wt % of silica having an averageparticle size of 1.0 μm and polybutylene terephthalate (Novadure 5010manufactured by Mitsubishi Engineering-Plastics Corporation, 50 wt %)were fed into an extruder. This extruder was a vent-type extruder(PCM-45 manufactured by Ikegai Corporation) and the mixture was extrudedfrom a T-die. The temperature of the extruder was exclusively set to280° C. The temperature of the resin that came out from the T-die wasalso 280° C. This was drawn 3.4 times in the longitudinal direction at68° C., then drawn 4.0 times in the transverse direction at 80° C. on atenter, and successively thermoset in a tenter at 240° C. for 1 sec, andfurther transversely relaxed by 5% at 160° C. to give a 25 μm thickfilm. While two melting point peaks were seen by DSC, the half valuewidth of the recrystallization peak at a decreasing temperature wasbroad and whitening was seen many times, possibly due to a somewhathigher temperature and the twin-screw extruder.

EXAMPLES 7, 8

In the same manner as in Example 1 except that the amounts of resinsfrom extruder I and extruder II were set to 85:15 and 20:80 (weightratio), respectively, a film was obtained.

EXAMPLE 9

In the same manner as in Example 1 except that polyhexamethyleneterephthalate having a reduced viscosity of 0.85 was used instead ofpolybutylene terephthalate and the ratio of the resins from extruder Iand extruder II was set to 90:10 (weight ratio), a film was obtained.

EXAMPLE 10

In the same manner as in Example 1 except thatpolyethylene-2,6-naphthalate having a reduced viscosity of 0.88 was usedinstead of polybutylene terephthalate and the ratio of the resins fromextruder I and extruder II was set to 90:10 (weight ratio), a film wasobtained.

EXAMPLE 11

In the same manner as in Example 2 except that the thermosetting wasperformed at 220° C., a film was obtained.

COMPARATIVE EXAMPLES 6, 7

In the same manner as in Comparative Example 4 except that onlypolybutylene terephthalate containing 0.1 wt % of silica or onlypolybutylene terephthalate containing 0.1 wt % of silica was used as astarting material, a film was obtained.

EXAMPLE 12

In the same manner as in Example 1 except that polyesters A, B and anorganic phosphorus compound were fed into an extruder such that thecomposition ratio became the same as in Example 1, and extruded underthe same temperature conditions as in Example 1, a film was obtained.This extruder had a screw of 90 mmφ, L/D=25, compression ratio 1.5, L/Dof compression part=12 with a compression part of a double flight type.While the temperature conditions were set the same as in Example 1, thetemperature of the resin that came out from a T-die was 257° C.

The production conditions of the polyester films produced in the aboveExamples 1-5, 7-12 and Comparative Examples 1-7 are shown in Table 1,and property values and test results are shown in Table 2.

TABLE 1 Extruder III Compression Cylinder- Resin Extruder I Extruder IIKind ratio filter T-die temperature TS Example 1 starting PET/PBT/phosphorus I + II = 40/60 90φ material silica temperature 275° C.240° C. Uniaxial 1.5 260° C. 255° C. 255° C. 150° C. Example 2 startingPET/ PBT/phosphorus/ I + II = 40/60 90φ material silica talc temperature275° C. 240° C. Uniaxial 1.5 260° C. 255° C. 255° C. 150° C. Comp. Ex. 1starting PET/ PBT/phosphorus I + II = 40/60 90φ material silicatemperature 275° C. 240° C. Uniaxial 1.5 285° C. 282° C. 266° C. 150° C.Example 3 starting PET/ PBT/phosphorus I + II = 40/60 90φ materialsilica temperature 275° C. 240° C. Uniaxial 4.0 260° C. 255° C. 263° C.150° C. Comp. Ex. 2 starting Not used Not used PET/PBT/ 90φ materialtemperature silica/phosphorus Uniaxial 4.0 260° C. 255° C. 265° C. 150°C. Example 4 starting PET/ PTT/phosphorus I + II = 40/60 90φ materialsilica temperature 275° C. 240° C. Uniaxial 1.5 260° C. 255° C. 260° C.150° C. Example 5 starting PET/ PBT/phosphorus I + II = 40/60 Staticmaterial silica mixer temperature 275° C. 240° C. 258° C. 255° C. 258°C. 150° C. Comp. Ex. 3 starting Not used Not used PET/PBT/silica 90φmaterial temperature 42 (with Uniaxial 4.0 290° C. 290° C. 290° C. 190°C. silica)/58 Comp. Ex. 4 starting Not used Not used PET/PBT/ 75φmaterial temperature Uniaxial 3.5 265° C. 265° C. 265° C. 150° C. Comp.Ex. 5 starting Not used Not used PET/PBT PCM45 material temperatureBiaxial 280° C. 280° C. 280° C. 240° C. vent Example 7 starting PET/PBT/phosphorus I + II = 85/15 90φ material silica temperature 275° C.240° C. Uniaxial 1.5 260° C. 255° C. 255° C. 150° C. Example 8 startingPET/ PBT/phosphorus I + II = 20/80 90φ material silica temperature 275°C. 240° C. Uniaxial 1.5 260° C. 255° C. 255° C. 150° C. Example 9starting PET/ PHT/phosphorus I + II = 90/10 90φ material silicatemperature 275° C. 210° C. Uniaxial 1.5 260° C. 255° C. 255° C. 150° C.Example 10 starting PET/ PEN I + II = 90/10 90φ material silicatemperature 275° C. 265° C. Uniaxial 1.5 265° C. 265° C. 265° C. 150° C.Example 11 starting PET/ PBT/phosphorus/ I + II = 40/60 90φ materialsilica talc temperature 275° C. 240° C. Uniaxial 1.5 260° C. 255° C.255° C. 220° C. Comp. Ex. 6 starting Not used Not used PET/silica 90φmaterial temperature Uniaxial 4.0 275° C. 275° C. 275° C. 150° C. Comp.Ex. 7 starting Not used Not used PBT/silica 90φ material temperatureUniaxial 4.0 260° C. 255° C. 255° C. 150° C. Example 12 starting Notused Not used PET/PBT/ 90φ material temperature silica/phosphorusUniaxial 1.5 260° C. 255° C. 257° C. 150° C. * In the Table, symbol Tsshows heat setting temperature.

TABLE 2 Half TmL TmH Tc2 value Whitening Can ηsp/c (° C.) (° C.) (° C.)width 280° C. 290° C. Hardness formability Example 1 0.94 221 252 1910.19 ⊙ ◯ 3H ⊙ Example 2 0.95 221 254 195 0.18 ⊙ ⊙ 3H ⊙ Comp. Ex. 1 0.79223 247 179 0.31 X X HB ⊙ Example 3 0.84 222 249 184 0.24 ◯ X 2H ⊙ Comp.Ex. 2 0.88 223 252 176 0.39 X X  H ⊙ Example 4 0.88 231 250 186 0.22 ◯ Δ2H ⊙ Example 5 0.93 221 253 191 0.19 ⊙ ◯ 3H ⊙ Comp. Ex. 3 0.75 Not seen251 166 0.41 X X HB ⊙ Comp. Ex. 4 0.79 220 250 179 0.32 Δ X  H ⊙ Comp.Ex. 5 0.82 218 250 175 0.37 X X 2H ⊙ Example 7 0.83 220 251 197 0.12 ⊙ ⊙3H ◯ Example 8 0.95 219 252 191 0.19 ⊙ ◯ 2H ⊙ Example 9 0.82 186 250 1840.23 ◯ X  H ◯ Example 10 0.88 254 260 183 0.24 ◯ X 3H ◯ Example 11 0.95221 253 194 0.18 ⊙ ⊙ 3H ◯ Comp. Ex. 6 0.71 Not seen 256 199 0.11 ⊙ ⊙ 3HX Comp. Ex. 7 0.95 223 Not seen 167 0.31 X X 3H ◯ Example 12 0.81 219249 181 0.25 ◯ X  H ◯

INDUSTRIAL APPLICABILITY

According to the present invention, a polyester film superior inmechanical characteristic and design property and having highcrystallinity, which can be used alone or adhered to a metal sheet andwhich is free of whitening even when the film is heat treated at near ornot lower than the melting point can be obtained, and further, apolyester film having such superior resistance to whitening (designproperty), which is not easily damaged, can be obtained.

The polyester film of the present invention can be applied as a film forvarious food packagings, general industrial purposes, optical use,electric material and form processing, and is particularly suitable forgeneral packaging, antistatic purpose, gas barrier, metal lamination,heat sealing, antifog, metal vapor deposition, easy tearing, easyopening, bag making and packaging, retort packaging, boil packaging,drug packaging, easy adhesion, magnetic recording, capacitor, inkribbon, transfer, adhesive label, stamping foil, gold and silverthreads, tracing material, release, shrinkable film and the like, andparticularly preferable for metal lamination (constituent material forfilm laminated metal sheet).

This application is based on a patent application No. 5098/2002 filed inJapan, the contents of which are hereby. incorporated by reference.

1. A film made from a polyester resin composition comprising 10-90 wt %of polyester (A) comprising ethylene terephthalate as a main constituentcomponent and 90-10 wt % of crystalline polyester (B) different fromsaid polyester (A), which film shows a half value width ofrecrystallization peak obtained by a differential scanning calorimeter(DSC) by lowering temperature of not more than 0.22; wherein crystallinepolyester (B) is a polyester selected from polybutylene terephthalate(PBT) and polytrimethylene terephthalate (PTT).
 2. The polyester film ofclaim 1, wherein a peak temperature (Tc2) of the recrystallization peakis not less than 180° C.
 3. The polyester film of claim 1, wherein thepolyester film has a reduced viscosity of not less than 0.80.
 4. Thepolyester film of claim 1, wherein the process of forming the filmcomprises: individually melting polyester A and polyester B in separateextruders; and mixing polyester A and polyester B in a mixing extruderafter melting polyester A and polyester B; wherein the mixing extruderhas a compression ratio of 1.1 to 3.8 and a ratio of an effective length(L) to an average outer diameter (D) of a screw of the mixing extruder(L/D) of 20 to 35; and the forming temperature in the mixing extruder isnot more than 265° C.
 5. The polyester film of claim 1, wherein theprocess of forming the film comprises: individually melting polyester Aand polyester B in separate extruders; and mixing polyester A andpolyester B in a mixing extruder after melting polyester A and polyesterB; wherein the mixing extruder has a compression ratio of 1.1 to 3.8 anda ratio of an effective length (L) to an average outer diameter (D) of ascrew of the mixing extruder (L/D) of 20 to 35; and the formingtemperature in the mixing extruder is not more than 265° C.; and whereinthe polyester resin composition further comprises an organic phosphorouscompound.
 6. The polyester film of claim 1, further comprising silica.